This application claims the benefit of Korean Patent Application No. 2002-18500 filed on Apr. 4, 2002, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device, and more particularly, to an organic electroluminescent device having an improved efficiency of injecting electrons from a cathode to an organic layer.
2. Description of the Related Art
Recently, much attention has been paid to electroluminescent devices because they have advantageous features suitable for next generation display devices, such as a wide viewing angle, a high contrast ratio and a high response speed. In particular, organic electroluminescent devices using organic materials as raw materials for light emitting layers have advantageous characteristics, including a high luminance, a low driving voltage, a fast response rate, ability of producing a wide range of colors, compared to inorganic electroluminescent devices using inorganic materials as raw materials for light emitting layers.
An organic electroluminescent (EL) device is basically configured such that an anode is formed on a substrate, and a hole transport layer, an emitter layer, an electron transport layer, and a cathode are sequentially stacked on the anode. Here, the hole transport layer, the emitter layer and the electron transport layer are organic layers made of organic materials.
In the organic EL device having the above-described configuration, if a voltage is applied to the anode and the cathode, holes injected from the anode migrate to the emitter layer via the hole transport layer, and electrons from the cathode are injected into the emitter layer via the electron transport layer. The electrons and holes are recombined at the interface between the hole transport layer and the emitter layer (or the electron transport layer) to generate excitons. The generated excitons de-excite from an excited state to a ground state causing fluorescent molecules of the emitter layer to emit light, thus forming an image.
In the organic EL device driven as described above, to improve a low driving voltage characteristic and charge balance between electrons and holes, it is necessary to increase an efficiency of injecting electrons from a cathode into an organic layer, e.g., an electron transport layer, from a cathode. Conventional methods for increasing such injection efficiency have been proposed in U.S. Pat. Nos. 5,429,884, 5,059,862 and 4,885,211, describing use of an alkali metal having a low work function, e.g., lithium or magnesium, codeposition of an alkali metal and a metal such as aluminum or silver, and use of alloys of an alkali metal and a metal such as aluminum or silver, respectively.
However, the conventional methods have several problems. That is, since the metal that has a low work function is very unstable and highly reactive, use of the metal is disadvantageous in view of the processibility and the stability of EL device. In the case of codeposition, it is substantially difficult to control the mixture ratio of materials for forming an alloy. Also, use of alkali metals deteriorates reproducibility.
Other techniques for increasing the electron injection efficiency have been proposed in U.S. Pat. Nos. 5,776,622, 5,776,623, 5,937,272 and 5,739,635, and Appl. Phy Lett. 73 (1998) 1185, in which an electron injection layer containing inorganic materials such as LiF, CsF, SrO or Li2O, is formed between a cathode and an organic layer to a thickness of 5 to 20 xc3x85. However, according to these techniques, inorganic material film formation requires an extremely high temperature, and it is quite difficult to form a thin film having a uniform thickness of 5 to 20 xc3x85.
Recently, another method for increasing an electron injection efficiency has been proposed in which an alkali metal acetate, e.g., CH3COOLi or C6H5COOLi, is formed between a cathode and an organic layer to a thickness of 5 to 40 xc3x85. This method also has a problem in that it is difficult to form a thin film having a uniform thickness of 5 to 40 xc3x85, which is not suitable for large-area deposition.
As described above, various attempts for enhancing electron injection have conventionally been made, but no satisfactory technique has been found yet in view of processibility, driving voltage and efficiency.
To overcome the above and/or other problems, the present invention provides an organic EL device with a reduced driving voltage, enhanced luminance and improved color coordinate characteristics by increasing an efficiency of injecting electrons from a cathode into an organic layer.
In an aspect of the present invention, an organic EL device includes a cathode, an anode, and organic layers formed between the cathode and the anode. In the organic EL device, the cathode includes a first cathode containing aluminum, and a second cathode disposed under the first cathode, formed in the vicinity of an organic layer and containing a metal having a work function of 3.5 to 5.0 eV, and metallic carboxylate represented by:
RCOOMxe2x80x83xe2x80x83Formula 1 
wherein M is an alkali metal or alkaline-earth metal, and R is a C1-C10 alkyl or C6-C20 aryl.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.